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High power laser system with built-in dynamic beam shaping capabilities

The PhD project investigated a method with which the power handling capacities of the modern spatial light modulators can be bypassed. The basic concept revolved around using the spatial light modulator to pre-shape the laser beam which is then to be amplified to a much higher level. In the final design, the spatial light modulator injects a phase modulated signal into an array of high power fibre amplifiers. This arrangement is already widely used in a scheme called coherent beam combining; here, the optical phases are synchronized and the beams from each source combine into a single powerful coherent beam. In this project, it is shown that by carefully selecting the phase distribution the spot shape can be arbitrarily selected.

Student

Jiho Han

Supervisors

Prof Bill O'Neill 

Advisor

Prof Ian Hutchinson

Introduction

Traditionally, a laser beam shaping problem is defined as redistribution of the intensity profile into another more desirable profile. Attempt goes as far back as when Frieden (1965) discussed a method for obtaining a top hat beam profile. Top hat beam profiles are still very useful today for applications such as laser machining, laser surface hardening, tattoo removal, keratectomy, and tissue welding. Today, beam shaping method are much generalised, and holography is able to arbitrarily transform a beam shape into another.

In addition, today, there exist dynamic beam shaping solutions such as Spatial Light Modulators(SLMs) or deformable mirror arrays. However, SLMs have a fairly modest power handling capabilities at around 15

W/cm^2 (Norton et al. 2010). Deformable mirrors can have up to 400

W/cm^2 of power handling capacities, but the degrees of freedom available for modulation are severely limited at around 10-100.

Therefore, it would seem that currently, high fidelity dynamic beam shaping and high power handling capabilities are incompatible. In this work, we focus on beam shaping, but the incompatibility between throughput and resolution is evident in other contexts: scanning a focussed laser beam offers high resolution while defocussed beam offers high through put, but the two incompatible!

 

Motivations

If the dynamic beam shaping capabilities of Spatial Light Modulators(SLMs) could be made compatible with today’s high power lasers, we may be able to process a 2D area through single exposure to a shaped high power laser beam, instead of relying on scanning. Process throughput would then be scaled with available average power rather than scanning speeds. The types of laser process that may benefit from this include laser material process (marking/engraving/machining), lithography, and selective laser melting for 3D printing.

 

Project Proposal

The currently perused concept is to take a low power beam, shape it, and then pass it through an appropriately designed optical amplifier.

Somewhat of a literal translation of such an approach, using bulk amplifiers have been demonstrated in the past, but would usually suffer from generic issues of single pass bulk amplifiers, such as low gains, poor energy efficiency, and non-linearity. This project focus on a design for an optical amplifier that can offers compatibility between high powers and high resolutions.

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